1. Field of the Invention
The present invention relates to medical implants, and, more particularly, to medical implants having a bone and tissue ingrowth structure, and to a method of manufacturing the medical implants.
2. Description of the Related Art
Implant fixation via bone and tissue integration into a porous scaffold has been in development since the 1950s, when polyvinyl sponges were implanted into canines (Bryan, R. S., et al., “The Effect of Polyvinyl-Formal (Ivalon) Sponge on Cortical Bone Healing.” Proceedings of the Staff Meetings, Mayo Clinic, 33 (1958): 453-457). The early 1970s saw the development of sintered beads and titanium fiber metal, which are still in use in orthopaedic implants today. (Galante, J., Et al., “Sintered Fiber Metal Composites as a Basis for Attachment of Implants to Bone.” Journal of Bone and Joint Surgery Am, 563 (1971): 101-114).
In the Mid 1990s, a design was developed for porous scaffolds for tissue ingrowth. For example, U.S. Pat. No. 5,732,469 discloses a prosthesis for the replacement of hard tissues of human bones and joints formed by a porous lamination component of thin, metal layers, each of which have a different pore pattern. Further, U.S. Pat. No. 6,010,336 discloses a living body-supporting member having a porous surface layer formed of ceramic material. However, the scaffolds known in the art which are constructed to encourage bone ingrowth have reduced strength due to the low contact area between adjacent layers. More specifically, the weak points in laminate scaffolds known in the art are in the resulting layer interfaces between individual layers, especially in shear parallel to these interfaces. Accordingly, if the scaffold struts are too thin, the scaffold will not satisfy the necessary strength. Additionally, implants formed from the laminate of thin metal layers are costly to produce, since the scaffold's strength must be bolstered by increased minimum thickness of the layers.
What is needed in the art is a medical implant which has an improved strength, particularly shear strength in planes parallel to individual layers, and which may be manufactured in a cost-effective way.